Terahertz wave refers to an electromagnetic wave whose frequency is within a range of 0.1 THz to 10 THz The terahertz wave is excellent in permeability to a wide variety of substances including paper, wood, plastic, and the like as compared with far infrared rays which are electromagnetic waves of higher frequency bands; meanwhile, the terahertz wave is excellent in straight advancing property and resolution as compared with millimeter waves which are electromagnetic waves of lower frequency bands.
The characteristic absorption spectrum of a large number of substances including polymeric compounds, such as sugar and protein, are included in the frequency bands of terahertz waves. By applying a terahertz wave to an object and observing a resulting transmitted wave or reflected wave with these features exploited, an advantage is brought about. The shape or internal structure of the object, the presence or absence of any defect or foreign matter, a difference in material or component, or the like can be non-destructively observed with the object kept in a permeable case. For this reason, it is expected that a wide range of terahertz wave applied science applicable to material inspection, structure inspection, chemical inspection, and the like will be implemented in the future.
In particular, by detecting a phase difference between a terahertz wave reflected at a measured object and a terahertz wave reflected at a metal or the like as a basis, an advantage is brought about. Surface asperities of the object, the thickness of a layer of a layered structure, an internal structure such as a void can be non-destructively observed.
By using terahertz waves of a plurality of frequencies to detect a phase difference between individual frequencies, the thickness of a complicated layer and a complicated internal structure can be observed.
By detecting a difference between the phase of a terahertz wave transmitted through a measured object and the phase of a terahertz wave propagated through a space without any object therein, an index of refraction of a material can be non-destructively observed.
However, the conventional terahertz wave technologies do not have a high output terahertz wave oscillator or a highly sensitive terahertz wave-detector and further methods for generating and detecting a plurality of frequencies of terahertz waves are limited. For this reason, some major problems arise. Practical measured objects are limited to thin films, such as painted films on paintings and works of art, foamed materials excellent in terahertz wave permeability, and the like and it takes a long time for measurements. Because of these problems, it used to be difficult to introduce a terahertz wave technology into a product inspecting apparatus or the like requiring high-speed and high-accuracy measuring performance.
As a conventional terahertz wave phase difference measuring system, for example, a method of generating and detecting a terahertz pulse wave described in Patent Literature 1 is known. Ultrashort pulsed light generated by a femtosecond laser is launched into a photoconductive antenna for generation to generate a terahertz pulse wave. The terahertz, pulse wave and the ultrashort pulsed light are substantially simultaneously launched into a photoconductive antenna for detection. By observing currents produced at the photoconductive antennas, the amount of terahertz pulse waves transferred between the two photoconductive antennas is obtained.
Using this method, a terahertz pulse waveform reflected at a measured object and a terahertz pulse waveform reflected at a metal plate as a reference are observed and delay times of the respective pulse waveforms are calculated. A phase difference between the terahertz waves can be thereby obtained. Shape and roughness on the surface of a measured object and the thickness and index of refraction of each layer as for a layered structure can be measured.
As described in Patent Literature 2, a method of generating a terahertz wave of a narrow hand and detecting the same with a terahertz wave detector of a wide band is known.
In addition, a method described in Non-patent Document 1 is known in which method a nonlinear optical crystal is used both for terahertz wave generation and for terahertz wave detection to achieve generation of a terahertz wave of a narrow band and a high intensity and detection of a terahertz wave with high sensitivity.